Method and Apparatus for Improved Reduced Load Operation of Steam Turbines

ABSTRACT

An apparatus and method for improving reduced-load operation of steam turbines. The apparatus may include a number of low pressure sections and a flow control system operable to limit the flow of steam to at least one of the number of low pressure sections. The method may include providing a number of low pressure sections, providing a flow control system operable to limit the flow of steam to at least one of the number of low pressure sections, and operating the flow control system to limit the flow of steam to the at least one of the number of low pressure sections when the load is below a predetermined value.

TECHNICAL FIELD

The present application relates generally to steam turbines and more particularly to the improved performance of steam turbines at reduced-load, part-load, or off-design operating conditions.

BACKGROUND OF THE INVENTION

Steam turbines are mechanical devices that convert the thermal energy of pressurized steam into rotational mechanical work. In order to improve thermal efficiency, large steam turbines generally include several sections, such as a high pressure section, an intermediate pressure section, and a low pressure section. Within each section, multiple stages may be used in the expansion of the steam.

The sections of large steam turbines possess various design characteristics so as to permit the extraction of the largest possible amount of energy from the expansion of steam through the turbine sections. It is common practice to introduce initially high pressure steam into a high pressure section of the steam turbine. Steam exiting the high pressure section of the turbine is then directed to a reheater before being introduced into a low pressure section of the turbine, where it continues its expansion. In many turbines the steam passes through an intermediate pressure section before being introduced into a low pressure section.

Because of the increase in volume as the steam expands through the stages of each section, the annulus area of each stage must increase in order to convert efficiently the thermal energy of the steam to rotational mechanical work. This is accomplished by lengthening the turbine blades or buckets from stage to stage, by increasing the diameter of the rotor upon which the blades are mounted, by adding two or more flow paths in parallel, or combinations thereof.

When two flow paths of a section of a turbine are in parallel, this is known as a double-flow section. It is a common practice to have one or more of the sections configured in a double flow arrangement in which steam entering a middle portion of the section encounters a diverging flow path. Following entry into this middle portion of one of the turbine sections, the steam exits in opposite directions with both flows directed to rotate the turbine shaft in the same direction. Thus, for example, steam entering from the top or bottom exits toward the left and right.

The choice of the single-flow or the double-flow low pressure section design may depend on the volume of steam designed to flow though the low pressure section at normal, full-load conditions. The best performance of low pressure steam path design may be achieved when steam velocity exiting the last stage buckets of the low pressure section is about 600 feet per second. Lower or higher velocities may degrade the performance of the steam turbine. In order to adjust the exit velocity of the low pressure section, the annulus area of the last stage buckets may be increased or reduced. If steam flow volumes are large enough, however, increasing the length of the turbine blades or the diameter of the rotor upon which the blades are mounted may not be feasible or desirable. Instead, a double-flow section may be used in order to increase the exit annulus area of the low pressure section. Similarly, if a single double-flow low pressure section does not provide sufficient annulus area to achieve adequate steam exit velocities, two or more double-flow low pressure sections may be used in parallel. In turbines having a number of double-flow low pressure sections, approximately equal portions of the steam flow may be introduced to each of the double-flow low pressure sections.

Steam turbines are designed to operate efficiently under normal, full-load operating conditions. In some circumstances such as a reduced demand for electricity, however, the steam flow to the turbine is reduced for economic reasons. When the flow is reduced below design conditions, the performance of the low pressure section of the steam turbine may drop significantly due to large separation at the hub of the last stage buckets. Exhaust hood performance also may decline. The low steam path efficiency and poor exhaust hood performance may cause poor turbine efficiency at reduced-load, low-load, or off-design operating conditions.

There is a desire, therefore, to provide a method and apparatus for improving the operation of steam turbines at reduced-load. Such a method and apparatus may improve the efficiency of steam turbines at such operating conditions.

BRIEF DESCRIPTION OF THE INVENTION

In one embodiment, the present application provides a method of operating a steam turbine at reduced load, including providing a number of low pressure sections, providing a flow control system operable to limit the flow of steam to at least one of the number of low pressure sections, and operating the flow control system to limit the flow of steam to the at least one of the number of low pressure sections when the load is below a predetermined value.

Another embodiment of the present application provides a method of operating a steam turbine at reduced load, including providing one double-flow low pressure section having two steam flow paths, providing a flow control system operable to limit the flow of steam to at least one of the two flow paths, and operating the flow control system to limit the flow of steam to the at least one of the two flow paths

A further embodiment of the present application provides a steam turbine. The steam turbine may include a number of low pressure sections and a flow control system operable to limit the flow of steam to at least one of the number of low pressure sections.

These and other features of the present application will become apparent to one of ordinary skill in the art upon review of the following detailed description when taken in conjunction with the several drawings and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a steam turbine having two double-flow low pressure sections in accordance with an embodiment of the present application as described herein.

FIG. 2 is a schematic view of a steam turbine having three double-flow low pressure sections in accordance with an embodiment of the present application as described herein.

FIG. 3 is a schematic view of a steam turbine having one double-flow low pressure section in accordance with an embodiment of the present application as described herein.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings, in which like numerals indicate like elements throughout the separate views, FIG. 1 shows a schematic view of a steam turbine 10 of an embodiment of the present application. Pressurized steam is supplied to the turbine 10 from a steam source 11, such as, for example, a boiler. Energy sources for generating steam may include, but are not limited to, fossil fuel, nuclear, geothermal, solar thermal electric, and biomass energy sources. The steam initially is introduced to a high pressure section 12 of the turbine 10. After the steam flow exits the high pressure section 12 of the turbine 10, the flow may pass to a reheater 13 where additional thermal energy may be added. In other embodiments the steam may be returned to a boiler where additional superheat may be added. In a further embodiment the steam may be introduced to a moisture separator to remove any moisture forming in the steam in the high pressure section of the turbine. In a still further embodiment, the steam may pass to an intermediate pressure section after the steam flow exists the reheater 13.

After the steam flow exits the reheater 13, a crossover pipe 14 may direct the steam flow to a first double-flow low pressure section 15 and a second double-flow low pressure section 16. A flow control system 17 may be provided, which is operable to limit the flow of steam to both of the two low pressure sections.

The flow control system 17 may include a first valve 18, located at the inlet of the first double-flow low pressure section 15, and a second valve 19, located at the inlet of the second double-flow low pressure section 16. In particular embodiments, the flow control system 17 may include any device that can regulate, halt, or control the flow of steam to at least one of the number of low pressure sections by closing or partially obstructing the flow path of the steam to at least one of the low pressure sections. For example, the flow control system 17 may include a butterfly valve, poppet valve, or a hinged lid or other movable part that closes or modifies the steam flow path. In another embodiment, the flow control system 17 may include only one valve disposed at the inlet of one of the low pressure sections. In a further embodiment, the flow control system 17 may include a valve disposed between the inlet of the first double-flow low pressure section 15 and the inlet of the second double-flow low pressure section 16.

In operation at reduced-load, low-load, or off-design conditions, the performance of the low pressure section of the steam turbine 10 may drop significantly. In order to improve turbine efficiency at such conditions, the flow control system 17 is operable to limit the flow of steam to at least one of the number of low pressure sections. When the steam flow is reduced below design conditions or below a predetermined value, the flow control system 17 may limit the flow of steam to at least one of the two double-flow low pressure sections. The predetermined load at which the flow begins to be limited is a function of the particular design and operating conditions. In general, this load is selected such that the overall output of the low pressure section is maximized.

For example, the second valve 19 may be closed significantly to limit the flow to the second double-flow low pressure section 16. In particular embodiments, sufficient steam flow may be allowed to pass through the second valve 19 in order to prevent windage heating in the second double-flow low pressure section 16. Limiting steam flow to the second double-flow low pressure section 16 may cause a corresponding increase in flow to the first double-flow low pressure section 15, thereby increasing the efficiency of the first double-flow low pressure section 15. The increase in efficiency of the first double-flow low pressure section 15 increases the overall efficiency of the low pressure section of the steam turbine 10 at reduced-load, low-load, or off-design conditions.

In a particular embodiment, the flow control system 17 may be operated to direct a flow of steam to at least one of the low pressure sections sufficient to produce a steam exit velocity from the low pressure section of between about 500 feet per second and about 700 feet per second. The specific flow rate range may vary with the size and configuration of the turbine 10 as a whole. The relative flow of steam through the two double-flow low pressure sections may be adjusted using the flow control system 17 in order to maximize overall turbine efficiency.

After the steam exits each flow end of the first double-flow low pressure section 15 and the second double-flow low pressure section 16, it may discharged to a condenser 20 or otherwise used.

FIG. 2 shows a schematic view of a steam turbine 27 in accordance with an embodiment of the present application. Instead of the two double-flow low pressure sections of FIG. 1, the steam turbine of FIG. 2 may have three double-flow low pressure sections. Pressurized steam is supplied to the turbine 27 from the steam source 11. The steam initially is introduced to the high pressure section 12 of the turbine 27. After the steam flow exits the high pressure section 12 of the turbine 27, it may pass to the reheater 13 where additional thermal energy is added. After the steam flow exits the reheater 13, the crossover pipe 14 directs the steam flow to the first double-flow low pressure section 15, the second double-flow low pressure section 16, and a third double-flow low pressure section 21. The flow control system 17 may be provided to limit the flow of steam to each of the three low pressure sections. In another embodiment, the flow control system 17 may only be operable to limit the flow of steam to one or two of the number of low pressure sections. The flow control system 17 may include the first valve 18, located at the inlet of the first double-flow low pressure section 15, the second valve 19, located at the inlet of the second double-flow low pressure section 16, and a third valve 22, located at the inlet of the third double-flow low pressure section 21.

In operation at reduced-load, low-load, or off-design conditions, the flow control system 17 limits the flow of steam to at least one of the three double-flow low pressure sections. For example, the third valve 22 may be closed significantly to limit the flow to the third double-flow low pressure section 21. In particular embodiments, sufficient steam flow may be allowed to pass through the third valve 22 in order to prevent windage heating in the third double-flow low pressure section 21. Limiting steam flow to the third double-flow low pressure section 21 may cause a corresponding increase in flow to the first double-flow low pressure section 15 and the second double-flow low pressure section 16, thereby increasing the efficiency of the first double-flow low pressure section 15 and the second double-flow low pressure section 16. The increase in efficiency of the first double-flow low pressure section 15 and the second double-flow low pressure section 16 may increase the overall efficiency of the low pressure section of the steam turbine 27 at reduced-load, low-load, or off-design conditions.

In another embodiment, both the second valve 19 and third valve 22 may be closed significantly to limit the flow to the second double-flow low pressure section 16 and third double-flow low pressure section 21. In particular embodiments, sufficient steam flow may be allowed to pass through the second valve 19 and third valve 22 in order to prevent windage heating in the second double-flow low pressure section 16 and third double-flow low pressure section 21. Limiting steam flow to the second double-flow low pressure section 16 and third double-flow low pressure section 21 may cause a corresponding increase in flow to the first double-flow low pressure section 15, thereby increasing the efficiency of the first double-flow low pressure section 15. The increase in efficiency of the first double-flow low pressure section 15 may increase the overall efficiency of the low pressure section of the steam turbine 27 at reduced-load, low-load, or off-design conditions.

In a particular embodiment, the flow control system 17 may be operated to direct a flow of steam to at least one of the low pressure sections sufficient to produce a steam exit velocity from the low pressure section of between about 500 feet per second and about 700 feet per second. The specific flow rate range may vary with the size and configuration of the turbine 10 as a whole. The relative flow of steam through the three double-flow low pressure sections may be adjusted using the flow control system 17 in order to maximize overall turbine efficiency. After the steam exits each flow end of the first double-flow low pressure section 15, the second double-flow low pressure section 16, and the third double-flow low pressure section 21, it may be discharged to a condenser 20 or otherwise used.

FIG. 3 shows a schematic view of a steam turbine 28 having one double-flow low pressure section in accordance with an embodiment of the present application. Pressurized steam may be supplied to the turbine 28 from the steam source 11. The steam initially is introduced to the high pressure section 12 of the turbine 28. After the steam flow exits the high pressure section 12 of the turbine 28, it may pass to the reheater 13 where additional thermal energy is added. After the steam flow exits the reheater 13, the crossover pipe 14 may direct the steam flow to the first double-flow low pressure section 15. The double-flow low pressure section 15 may have a first steam flow path 23 and a second steam flow path 24. The flow control system 17 may be provided to limit the flow of steam to at least one of the two flow paths. The flow control system 17 may include a first valve 25, located at the inlet of the first flow path 23, and a second valve 26, located at the inlet of the second flow path 24.

In operation at reduced-load, low-load, or off-design conditions, the flow control system 17 limits the flow of steam to at least one of the two flow paths. For example, the second valve 26 may be closed significantly to limit the flow to the second flow path 24. In particular embodiments, sufficient steam flow may be allowed to pass through the second valve 26 in order to prevent windage heating in the second flow path 24. Limiting steam flow to the second flow path 24 may cause a corresponding increase in flow to the first flow path 23, thereby increasing the overall efficiency of the first double-flow low pressure section 15 at reduced-load, low-load, or off-design conditions.

In a particular embodiment, the flow control system 17 may be operated to direct a flow of steam to at least one flow path sufficient to produce a steam exit velocity from the flow path of between about 500 feet per second and about 700 feet per second. The specific flow rate range may vary with the size and configuration of the turbine 28 as a whole. The relative flow of steam through the two flow paths may be adjusted using the flow control system 17 in order to maximize overall turbine efficiency. After the steam exits each flow end of the first double-flow low pressure section 15, it may be discharged to a condenser 20 or otherwise used.

It should be understood that the foregoing relates only to the preferred embodiments of the present application and that numerous changes and modifications may be made herein without departing from the general spirit and scope of the invention as defined by the following claims and the equivalents thereof. 

1. A method of operating a steam turbine at reduced load, comprising: providing a plurality of low pressure sections; providing a flow control system operable to limit the flow of steam to at least one of the plurality of low pressure sections; and operating the flow control system to limit the flow of steam to the at least one of the plurality of low pressure sections when the load is below a predetermined value.
 2. The method of claim 1, wherein the step of providing a plurality of low pressure sections comprises providing two double-flow low pressure sections.
 3. The method of claim 1, wherein the step of providing a plurality of low pressure sections comprises providing three double-flow low pressure sections.
 4. The method of claim 1, wherein the step of providing a flow control system operable to limit the flow of steam to at least one of the plurality of low pressure sections comprises providing at least one valve disposed at the inlet of the at least one of the plurality of low pressure sections.
 5. The method of claim 2, wherein the step of providing a flow control system operable to limit the flow of steam to at least one of the plurality of low pressure sections comprises providing a valve disposed between the inlet of a first double-flow low pressure section and the inlet of a second double-flow low pressure section.
 6. The method of claim 3, wherein the step of providing a flow control system operable to limit the flow of steam to at least one of the plurality of low pressure sections comprises providing a first valve disposed at the inlet of a first double-flow low pressure section and a second valve disposed at the inlet of a second double-flow low pressure section.
 7. The method of claim 1, wherein the flow control system provides a flow of steam to each of the plurality of low pressure sections to prevent windage heating.
 8. The method of claim 1, further comprising operating the flow control system to direct a flow of steam to at least one of the plurality of low pressure sections sufficient to produce a steam exit velocity of between about 500 feet per second and about 700 feet per second from the at least one of the plurality of low pressure sections.
 9. A method of operating a steam turbine at reduced load, comprising: providing one double-flow low pressure section having two steam flow paths; providing a flow control system operable to limit the flow of steam to at least one of the two flow paths; and operating the flow control system to limit the flow of steam to the at least one of the two flow paths.
 10. A steam turbine, comprising: a plurality of low pressure sections; and a flow control system operable to limit the flow of steam to at least one of the plurality of low pressure sections.
 11. The steam turbine of claim 10, wherein the plurality of low pressure sections comprises two double-flow low pressure sections.
 12. The steam turbine of claim 10, wherein the plurality of low pressure sections comprises three double-flow low pressure sections.
 13. The steam turbine of claim 10, wherein the flow control system comprises at least one valve disposed at the inlet of the at least one of the plurality of low pressure sections.
 14. The steam turbine of claim 11, wherein the flow control system comprises a valve disposed between the inlet of a first double-flow low pressure section and the inlet of a second double-flow low pressure section.
 15. The steam turbine of claim 12, wherein the flow control system comprises a first valve disposed at the inlet of a first double-flow low pressure section and a second valve disposed at the inlet of a second double-flow low pressure section.
 16. The steam turbine of claim 10, wherein the flow control system is operable to provide sufficient flow of steam to each of the plurality of low pressure sections to prevent windage heating.
 17. The steam turbine of claim 10, wherein the flow control system is operable to direct a flow of steam to at least one of the plurality of low pressure sections sufficient to produce a steam exit velocity of between about 500 feet per second and about 700 feet per second from the at least one of the plurality of low pressure sections.
 18. The steam turbine of claim 10, further comprising a moisture separator, wherein the moisture separator provides the flow of steam to the plurality of low pressure sections.
 19. The steam turbine of claim 10, further comprising a crossover pipe, wherein the crossover pipe provides the flow of steam to the plurality of low pressure sections. 